Triax
[G. Bogner, Transmission of electrical energy by superconducting cables, in “Superconducting Machines and Devices”, Ed. S. Foner and B. Schwartz (Plenum Publishing Co., 1974), pp. 430-431] and [T. Tanaka, A. Greenwood, Advanced Power Cable Technology—Volume II: Present and Future, (1983, CRC Press, Boca Raton, Fla.), pp. 242-259] describe a triaxial superconducting cable with three concentric phase conductors. The superconductors are applied to the surfaces of cooling ducts suspended in vacuum. The middle conductor is described as a double conductor, coating both sides of an annular cooling duct. The author indicates the difficulty to control the current distribution in the two conductors of Phase 2. This current distribution is desirable to eliminate eddy-current losses in the cooling duct inside phase 2. A common thermal insulation (cryostat) is concentric to the conductors. The electrical insulation is achieved by solid spacers, reflective foils and vacuum.
DE-43 40 046 describes a tri-axial AC cable with three concentric conductors and a common screen. The cable assembly is concentric with a thermal insulation. There are concentric central and annular cooling channels. In this way uniform cooling is achieved around the cable. The three phase conductors are made from ribbons of BiSrCaCuO in silver sheath. The cooling fluid in the form of liquid nitrogen can flow in the central and ring-shaped concentric cooling channels. The phase conductors are separated by a 10-50 mm thick layer of PE or polypropylene ribbons that form the electrical insulation. The insulation thickness between the third phase and the screen is only 60% of the insulation thickness between two phase conductors. The cooling medium travels out in the central cooling channel (50-200 mm Ø) and return in the ring-shaped annular cooling channel (150-500 mm). Due to the radial heat exchange between these two flows, the far end of this cable will experience an extreme temperature excursion, exceeding the temperature difference between the go and return flows. Some difficulty in manufacturing and transport would be expected due to a large dimension and weight of the cryostat with the cable assembled concentrically inside. The production and installation unit length of the electrical phase conductors becomes limited by the unit length of the cryostat. There is a technical difficulty in achieving the centring of the cable assembly while the cryostat is manufactured over the cable conductor assembly. However, there may have been a drive towards achieving centricity because in the case of a current imbalance, centricity is a cause of reduced eddy current loss in comparison with the eccentric position. In the described design, conductors consisting of BSCCO would overheat if exposed to over-currents frequently occurring in real power networks. If the silver sheath is made thicker, to act as a stabilizer, this cable design would become unattractively expensive.
Coaxial
Sato et al. (IEEE Transactions on Applied Superconductivity, Vol. 7, No. 2, 1997, pp. 345-350) describe a 3 phase HTS cable using BSCCO material for the conductors in a parallel, non-concentric configuration. Each phase comprises a former, a HTS-conductor, a LN2 impregnated PPLP-insulator and a HTS insulation screen. Each electric phase has its own LN2 centrally located cooling channel as well as a common ‘outer’ cooling channel formed by the corrugated tube system constituting the cryostat and surrounding the 3 separate phases. This design is dedicated to three-phase AC systems and requires HTS material for six times the ampacity (current rating) of one phase (three phases and three screens). In the case of a bi-polar DC system, a conform “two-phase” system would require HTS material for four times the phase ampacity (two phases and two screens), i.e. the described design principle requires 2N times the phase ampacity. The present invention requires from N times, up to N+1 times the phase ampacity of HTS materials, where N is the number of phases. The present invention only requires one former per N-phase system, N<N+1<2N for N>1.
Leghissa et al. (IEEE Transactions on Applied Superconductivity, Vol. 9, No. 2, 1999, pp. 406-411) describe the development of a 110 kV/400 MVA coaxial 1-phase HTS model cable. The conductors are made of BPSCCO multifilament tapes and are electrically insulated by a high-voltage insulation of LN2-impregnated synthetic tapes. The cable has a co-axial superconducting shield conductor. The cable core is housed in a flexible cryostat consisting of superinsulated corrugated tubes, and rests on the bottom of the inside of the cryostat without centering arrangements. A three-phase system can be constructed from three such single-phase, coaxial cable conductors inside a common cryostat or each in a separate cryostat. The cable is cooled with a closed-cycle LN2 system.
Thermal Contraction Management
JP-09-134624A discloses a method of manufacturing a superconductive cable wherein the problem of managing the length change of the cable during large temperature changes (such as from room temperature to a low operating cryogenic temperature or vice versa) is solved in that the cable is fed into a thermal envelope during production and simultaneously cooled by liquid Nitrogen, the cable following a linear path in the thermal envelope. During a subsequent return to room temperature, the cable is confined to the same length and allowed to expand resulting in a nonlinear (e.g. snaking) path in the thermal envelope.